Max depth with pony

[NeedABiggerBoat]

Hey guys, I was wondering what are the deepest depths you could return to the surface in an OOA emergency with a 19 cu ft pony, and a 30 cu ft pony respectively. I currently use a 40 cu ft pony, and although I've never needed it, I am interested in sliming down to something a little smaller. I know I could return to the surface from the limit of recreational scuba with my 40, would it be possible with a 30 or 19? Thanks!

 

[arkstorm]

The answer probably depends on an individual's SAC rate but if we're not talking deco or any mandatory stops, I imagine I could safely escape to the surface from 130' on a 19cf and maybe even get in a safety stop.

 

[waterpirate]

Does your bailout pony include or exclude gas for buddy? I used a 30 pony down to 130 when diving solo and it was adequate for bailout plus stops. your milage may vary.
Eric

 

[Captain12Pk]

First I would suggest sticking with the 40. I've never once come up from a dive thinking "Wow, I just had way too much air for that dive". However, if you were to find yourself in an OOA situation, you may find yourself thinking the opposite. Besides, as your diving progresses, you may be able to adapt the 40 to other uses.
Second, knowing how long a particular volume of air will last at a particular depth is a gas management skill. A skill apparently not taught in OW classes anymore. I would also suggest finding a qualified instructor to go over this with you in detail, but just for fun, lets have a whack at it.
For illustration purposes, lets use 1cf/min in an OOA situation.
@ 33fsw (2 ata)one diver would consume 2 cf/min. If two divers are breathing off the tank... 4 cf/min.
@66 fsw (3 ata)one diver would consume 3 cf/min. If two divers are breathing off the tank... 6 cf/min.
See how this works?
Dont forget to add in safety stops as needed.

 

[Dave1981]

assuming a pretty good SAC of 1cu.ft/min during an emergency, a 3 min stop, an extra minute at the bottom to sort out your problems and 30ft/min ascent then that's 8mins to surface from 130'

lets conservatively say the ave depth for the ascent then is 65' or 3ata. That's 21cu.ft for you alone. If you're overweight/unfit or panicking then you can reasonably add a good whack on to that.

 

[GrumpyOldGuy]

My 2 cents...

If you are using a pony, you should be practicing with it and thus know exactly how much air you need and can just add a generous safety margin to allow for stress (at least 50%).

If you are not practicing with it, then you may be lugging around a giant paper weight that can give you a false sense of security. People can guess, use rules of thumb, etc... but if you are going to lug it around you should take the time to know if it is the correct size, not guess.

 

[kanonfodr]

Well, without knowing anything about your personal RMV, I'm going to use some generic numbers and do some math.

All dives will assume a 30 fpm ascent rate, with a deep stop for 1 minute at half your max depth and then a 3-minute safety stop at 10 feet, with a 10 fpm ascent to the surface. Diver RMV will be using a "stressed" rate of 1.0 cu/ft min at the surface.

100 feet:

1 minute at 100 feet to sort out the problem on pony gas: 4 ata X 1cfm x 1 min = 4.0 cu ft

ascent from 100 to 50 feet using average depth (75 feet): 3.27ata x 1 cfm x 1.66 min= 5.42 cu ft

1 min at 50 feet (deep stop): 2.5ata x 1cfm x 1 min = 2.5 cu ft

ascent from 50ft to 15ft using average depth: 2.0ata x 1cfm x 1 min = 2 cu ft

3 min safety stop at 15 ft: 1.5ata x 1cmf x 3 min = 4.5 cu ft

ascent to surface from 15 feet at 10 fpm using average depth: 1.25ata x 1cfm x 1.5 min = 1.875 cu ft

And the grand total: 20.295 cu ft required for ascent under emergency conditions.

Now for 130 feet

1 min at 130 feet on pony to sort out the problem on pony gas: 5ata x 1cfm x 1min = 5 cu ft

ascent from 130 to 65 feet using average depth: 4ata x 1cfm x 2 min = 8 cu ft

deep stop at 65 feet for 1 minute: 3ata x 1cfm x 1 min = 3 cu ft

ascent to 15 feet using average depth: 2.25ata x 1cfm x 1.5min = 3.375 cu ft

3 min safety stop at 15 ft: 1.5ata x 1cmf x 3 min = 4.5 cu ft

ascent to surface from 15 feet at 10 fpm using average depth: 1.25ata x 1cfm x 1.5 min = 1.875 cu ft

And for the grand total: 25.75 cu ft required for ascent under emergency conditions.

Conclusion: While your 19 would have enough gas for an abbreviated ascent from 100 feet or shallower, it would be woefully inadequate for anything deeper. The 30, however would be a good option for either scenario IMO.

Peace,
Greg

 

[AndyNZ]

A 30 cu ft is not that much smaller than a 40 cu ft, really (I have both). In some cases a 19 cu ft may be big enough, in others it may not.

As Dirty Harry would say..... do you feel lucky, punk?

FWIW, if you are slinging a 19 cu ft tank, they actually sit really awkwardly because they are shorter. Because a 30/40 cu ft is longer the lower attachment is higher up the tank - this lets the tank swing up and tuck in behind your arm.

I've taught courses where students try different pony pottles - with no exception, every student has perferred the way a 30/40 cu ft tank sits over a 19 cu ft.

Different story if you are tank mounting it, mind you.

 

[String]

The max depth depends entirely on you. To be blunt if you're asking this question you dont really understand enough to be using a pony (it hints you already use one without knowing its limitations).

You need to know your own SAC, your own stressed SAC rate, the capacity of the pony and only then can you work out the maximum depth it'll get you out of jail from at your chosen ascent speed.

Not that i use one any more but i know for myself my old 3l bottle when full to 232 bar would get me out from about 33m with a normal ascent rate and 3 minute stop at 5m.

 

[Buoyant1]

Just stick with the 40...you'll thank yourself in the long run...I don't even know mine's there half the time.